Hittite Journal of Science and Engineering, 2016, 3 (2)8-
ISSN NUMBER: 2149-2123
DOI: 10.17350/HJSE19030000036
Classification and Rating of Inclusions in Steel Using an Image
Analysis Software
Oktay Elkoca
ArcelorMittal Global R&D Center, East Chicago, IN, USA
A B ST R AC T
I
nclusions play an important role in the performance of steel products. In this respect, they
should be accurately characterized in steels. Developing computer technology and softwares
have been allowed to evaluate the inclusion content of a steel products by classifying and rating
numerous inclusions in a large number of fields through optical microscope. However, due to
the difficulties encountered in classification, it still needs experienced operators’ intervention,
and advanced tools like SEM-EDS for accurate results.
Keywords: Steel; Inclusion; Inclusion rating; Image analysis; Metallography
Article History:
Received: 2016/09/29
Accepted: 2016/10/26
Online: 2016/12/31
Correspondence to: Oktay Elkoca,
ArcelorMittal Global R&D Center,
3001 East Columbus Drive, East
Chicago, IN 46312-2939, USA
Tel: +1 (312)7720874
E-Mail: oelkoca@gmail.com
INTRODUCTION
I
nclusions are non-metallic particles embedded in
steel matrix. The particles usually are compounds
such as oxides, sulphides, and silicates, but may be
any substance insoluble in the matrix [1]. Sims [2]
classified nonmetallic inclusions based on their origin
as endogenous and exogenous. The endogenous
inclusions are formed by the reactions in liquid
metal in steelmaking process and their formation is
dictated either by additions to the liquid metal or by
changes in solubility during the solidification process.
Oxides and sulphides are examples of endogenous
non-metallic inclusions in steels. The exogenous
inclusions in steels, on the other hand, occur as a
result of trapping of slag, refractories, and oxidized
metal that the liquid metal comes in contact with
during the melting and casting process [2,3].
While inclusions are advantageous for certain applications such as machining and oxide dispersion strengthened steel alloys, under uncontrolled conditions they
can be deleterious to the performance of a steel product
[4]. Their origin, type, size, shape, number, and distribution may influence almost all properties of a steel such
as formability, machinability, weldability, fatigue, fracture, creep, corrosion, and toughness [4,5]. Therefore,
inclusions should be identified, classified, and rated prior to manufacturing processes of a steel product. Various techniques are available to monitor and characterize
the inclusion content of a steel. Microscopic technique
based on viewing a metallographically prepared sample
through an optical microscope is still very valuable and
prevailing one among all known methods. In this method, the inclusions in a polished sample are observed
through an optical microscope and classified and rated
according to a standard method. Standard reference
charts depicting a series of typical inclusion configurations (size, type, and number) in those standards have
been created for direct comparison with the microscopic field of view [6]. Therefore, rating practically can be
done in comparison with the standard charts such as
ASTM E45 [7]. In these charts inclusions are assigned
to a category based on similarities in morphology, i.e. by
shape, size, concentration, and distribution.
Inclusions are classified into four categories called as type based on their morphology. Each of them is
classified into two subcategories based on their width
or diameter. Although the categories contain chemical
names that imply knowledge of their chemical content,
the ratings are strictly based on morphology. In ASTM
E45, the four categories, or types, are partitioned into
severity levels based on the number or the length of the
particles present in a 0.50 mm2 field of view (Figure 1).
A-type and C-type inclusions are very similar
in size and shape. Therefore, they are distinguished
based on their color when viewed under the brightfield
illumination of an optical microscope, A-type as light
gray and C-type C as black. B-type inclusions consist of
at least three of round or angular oxide particles with
Epiphot 200 optical microscope (OM) at 100x magnification, and the detected inclusions were analyzed by Jeol
5600 JSM scanning electron microscope (SEM) equipped
with Oxford energy dispersive spectroscope (EDS). Clemex
Inclusion Rating (CIR) software integrated with the optical
microscope was used to discriminate, categorize, measure
and rate the inclusions found in the sample.
aspect ratios less than 2 that are aligned nearly parallel to
the deformation axis. D-type inclusions are globular in
shape.
The total analysis area was 376.48 mm2. Inclusions
were classified into four types and categorized into thin and
heavy series as shown in Figure 1. The results were expressed according to ASTM E45 Method A [7].
RESULTS AND DISCUSSION
Figure 1. Classification of inclusions according to ASTM E45 [7].
The first step for the automated inclusion rating system
is to adjust the light of optical microscope and to verify
the threshold. According to the gray level, the threshold
defines whether it is a sulfide or an oxide (Figure 2).
As mentioned above, a common method for the
analysis of inclusions is the comparison of the microscopic
fields with reference charts. Manual method is rather time
consuming, thus restricted with limited fields, thanks to
the image analysis softwares a large number of inclusions
in numerous fields can be rapidly classified and rated
according to their morphological differences through
optical microscope [8]. However, due to the difficulties in
distinguishing sulphide, silicate, and oxide inclusions it still
needs experienced operators’ intervention, and advanced
tools like scanning electron microscope (SEM) equipped
with energy dispersive spectrometry (EDS) analysis for
accurate results.
O. Elkoca/ Hittite J Sci Eng, 2016, 3 (2)87-90
MATERIAL AND METHODS
A sample was taken from a hot rolled S275 JRC steel sheet in 3.5 mm thickness, which is suitable for wheel manufacturing. In wheel steels, elongated inclusions have
a deleterious effect on the performance of rim forming
and electrical resistance welding, therefore, the sulphide
and alumina type inclusions are principally modified by a
Ca-Si treatment in steelmaking process. During the Ca-Si
treatment, alumina/silicate type inclusions are converted
to molten calcium aluminates/silicates, which are globular in shape because of the surface tension effect at liquid
stage, and thus become harmless. The change in inclusion composition and shape is known as the inclusion
morphology control. Following steelmaking process, cast
and solidified metal is hot rolled into final sheet thickness.
The sample was mounted in bakalite so as to expose
the cross-sections parallel to rolling direction, ground with
180, 240, 320, 400, 600, and 1000 grit SiC papers, and polished with 9, 6, 3, and 1 μm diamond pastes on napless polishing cloths. Subsequently it was washed, rinsed with ethyl
alcohol, and dried with blown air.
Inclusion content of the sample was viewed by Nikon
88
Figure 2. According to defined gray level, inclusions are detected
as a sulfide or an oxide.
During thresholding, difficulties were encountered in
discriminating the thin sulphide, silicate, and oxide type
inclusions since thin B and D-type inclusions were often
confused with thin A-type inclusions (Figure 3a-c).
SEM-EDS analysis was performed on the selected inclusions to prevent any confusion. In this way, many thin
inclusions previously identified as B-type based on the
morphological appearance through optical microscope
were corrected as A-type with the aid of Mn and S peaks
(constituents of MnS) in the SEM-EDS spectrum (Figure 3b).
Additionally, the inclusions previously identified as
globular oxides were revealed as modified Ca-Al-O spinel
inclusions based on their SEM-EDS analysis (Figure 3d).
Following the results of SEM-EDS analysis, threshold levels
in Clemex CIR software were redefined and the incorrectly
identified inclusions were converted to true ones. As shown
in Figure 4, the squares of a specific color indicate the worst
fields (thin and heavy) for each category.
microscopic fields, whereby large amount of steel products
could be analyzed in terms of inclusion content. However,
due to the difficulties encountered in classification it still
needs experienced operators’ intervention, and advanced
tools like SEM-EDS for accurate results.
Figure 3 (a) elongated and (c) globular inclusions through optical microscope, and (b) and (d) their SEM-EDS analysis at red crosses “+” in micrographs
respectively.
To validate the results, the detected inclusions in the
worst fields were reviewed. If a dust residue or polishing
scratch remaining from the sample preparation process was
detected as an inclusion, it was removed from the results.
The results were expressed according to ASTM E45 Method A (Table 1).
Table 1. Clemex CIR report
CONCLUSION
It is well established that inclusions play an important role
in the performance of steel products, depending on
their type, size, shape, and distribution. In this respect,
they should be accurately characterized in steels. Thanks
to rapidly developing computer software technology,
it allows inclusions to be evaluated according to their
type, size, shape, and distribution in a steel by classifying
and rating of numerous inclusions in a large number of
1.
2.
3.
4.
5.
6.
Figure 4. Total analysis area.
89
Metallography and Microstructure, Metals Handbook, Vol. 9,
Ninth Ed., ASM (1985) p. 9.
Sims, C. E. Transactions of the Metallurgical Society of AIME,
215 (1959) 367-393.
Kissling, R. Non-Metallic Inclusions in Steels, The Institute
of Metals, London, (1978).
Rungta, R., Skidmore,A. J. and Buchheit, R. D. Inclusions:
Advantages, Disadvantages, and The Technological Trends,
Ed. by R. Rungta, World Materials Congress, Chicago, Illinois,
USA, Sept. 1988, ASM International, pp. 1-19.
Gladman, T., Holmes, B. and McIvor, I. D. The Effect of
Second Phase Particles on the Mechanical Properties of
Steel, Iron and Steel Institute Special Report 145, London,
(1971), pp. 68–78.
SS 111116, Steel-Method for Estimation of the Content
of
Non-metallic
Inclusions-Microscopic
MethodsJernkontoret’s Inclusion Chart II for the Assesment of
O. Elkoca/ Hittite J Sci Eng, 2016, 3 (2) 87-90
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90